1. Technical Field
This invention relates to diathermic devices used for therapeutic heating and, in particular, to systems for administering the application of diathermic energy in a controlled way.
2. Background Art
Resonant-circuit diathermic devices used for therapeutic heating (as opposed to non-resonant devices which have been eclipsed) represent a growing market. These devices have the ability to provide accurate warming of body areas without heating the intervening skin. U.S. Pat. No. 4,685,462, for example, describes warming of the body in the treatment of severe hypothermia, wherein a victim may be warmed internally in order to minimize the risk of cardiac arrest or skin burns. U.S. Pat. No. 6,094,599 describes the use of such a device in a controlled medical or therapeutic environment, such as a series of therapy sessions, to provide deep warming of injured tissue to enhance the therapeutic effect of improved blood flow. U.S. Pat. No. 5,160,828 teaches the use of body-conforming garments for the purpose of precisely controlling the duration and location of heating application during therapeutic sessions.
With reference to FIG. 1, prior art resonant assemblies rely upon the use of two inductively-coupled coil assemblies 2 and 4. These assemblies typically include a first source-side inductive coil (or primary coil or winding) 6 which is driven by a tuned RF source 8 and inductively-coupled to a second patient-side inductive coil (or secondary coil or winding) 10 which acts as a resonant applicator. Coil assembly 4 may include a tuning device 12, such as a variable capacitor, that facilitates tuning of coil assembly 4 into resonance with coil assembly 2 when coil assembly 4 is positioned to apply therapeutic heating to a patient (not shown).
Inductive coupling is achieved by placing primary winding 6 near a portion of the tuned secondary winding 10. The patient-side inductive coil (or secondary winding) 10 is placed adjacent the source-side coil (or primary winding) 6 at a predetermined distance D from said source-side coil 6. Typically, the chosen distance D is some known small fraction of a wavelength in order to facilitate constructive wave induction such as is common in applications regarding the transfer of electromagnetic wave energy from one medium to another. The combination of the patient-side coil 10, together with the patient's body tissue and a suitable tuning device 12 (such as a variable capacitor), constitute a circuit. The variable capacitor 12 allows the resonant frequency of this circuit to be adjusted, or “tuned” to resonance for efficient delivery of power to the patient.
Careful placement of the coils 6 and 10 achieves desired coupling. By selecting an optimum turns ratio, the impedance of the tuned circuit (10, 12 and the patient's body tissue) can be transformed to a convenient value, such as 50 or 70 ohms, for connection to RF source 8.
However, the source-side coil 6 thus inductively coupled suffers from several disadvantages. For example, secondary coil 10 is required to encompass a body part such as an arm or leg and is therefore of minimum size. Being conductive because of its inductive properties, secondary coil 10 must also be made of metal wires, which add weight. The weight and size inherent to these requirements hampers mobility and flexibility, from the points of view of both the patient and the therapy administrator.
Moreover, the wire windings of secondary coil 10 are inherently sensitive to spacing. Specifically, the windings of secondary coil 10 are desirably constructed in a periodic way, that is to say that the distance between consecutive turns of the winding of secondary coil 10 is desirably kept as constant as possible at a predetermined separation distance for optimal power transfer as is known in the art.
Furthermore, in order to be used in a point-of-treatment environment, secondary coil 10 must be portable and operable by health personnel with no guaranteed knowledge of its internal electrical function.
Yet another disadvantage is the non-uniform distribution of magnetic field in the secondary coil assembly 10 due to the effects of induction. Still a further disadvantage is the incidental and undesired capacitive coupling between coils 6 and 10 which “unbalances” secondary coil assembly 4 by forcing the end of secondary coil 10 near primary coil 6 to be closer to the normally near ground potential of RF source 8.
While inductive coupling of coils 6 and 10 suggests that it is balanced since current flowing in secondary coil 10 is caused by current in primary coil 6 and not by the voltages present relative to ground by either end of primary coil 6, in reality inductive coupling of coils 6 and 10 is not well balanced. This is because there is substantial stray capacitive coupling from primary coil 6 to whatever portions of secondary coil 8 are located nearby, which stray capacitive coupling adds a current path from primary coil 6 to secondary coil 10.
If primary coil 6 is driven by an unbalanced shielded co-axial cable 30, as is normally the case, one end of primary coil 6 is connected to ground G (as shown in FIG. 1) and the other end is near ground potential because primary coil 6 represents a low impedance path to ground G.
U.S. Pat. No. 6,853,865 suggests adding a balanced transformer (a balun) before primary coil 6 to convert the drive signal to primary coil 6 to an unbalanced feed relative to ground. This has proven to be less than ideal in practice because building a balanced balun of sufficiently high output impedance, relative to the very high impedance of the resonant secondary coil 10, is not practical. It has also been suggested to place primary coil 6 at the center of secondary coil 10. This is also difficult to do because primary and/or secondary coils of diathermic applicators can take on an asymmetric shape(s). Furthermore, primary and/or secondary coils designed to conform to body part shapes, as is usually the case, are not perfect cylinders. Still further, the human body presents stray capacitances to ground, which make the physical center of the secondary coil an uncertain effective ground point. Furthermore, inductive coupling between primary and secondary coils suffers from being subject to subtle details of wire placement and it is difficult to retain this exact placement over time. Also, inductive coupling cannot be done in a well-balanced manner because of uncertainty of the virtual center of the secondary coil. The prior art has not identified the source of this problem.
In some ways unbalanced inductive coupling resembles the problem in the field of audio amplifiers of common-mode voltage caused by poor grounding leading to a 60-cycle hum. U.S. Pat. No. 4,979,218 teaches common-mode rejection to overcome poor grounding in audio amplifiers. However, audio amplifiers do not suffer from the same kind of unpredictable variation in output circuit dynamics since they do not radiate at frequencies sufficient to inadvertently incorporate surrounding objects into their resonant circuit.
Also, the problem of unbalanced inductive coupling cannot simply be solved by providing shielding, such as in the case of a microwave oven (U.S. Pat. No. 4,431,888) because the element being heated by the present invention (in this case the patient) has a path to ground which is not contained.
U.S. Pat. No. 4,996,484 teaches noise cancellation to remove power line noise from geophysical equipment, but requires the use of complicated electronics such as phase-locked loops. U.S. Pat. No. 4,837,514 teaches tracking and digitally removing noise.
However, neither the balun nor any other feature of the prior art is capable of overcoming the leakage problem due to the unintentional unbalancing of the secondary coil and attendant de-tuning of the entire resonant circuit unavoidably caused by the patient.
While capacitive coupling networks for signal transmission are generally known, capacitive coupling networks designed to overcome the unpredictable, adverse and stray capacitances that couple through leakage into a patient's body are not known in the prior art.
Consequently, there exists a need to provide a compact, robust system that stably applies tuned-power through a patient-side coil for the purposes of therapeutic treatment.